1. Field of the Invention
The present invention relates to imaging devices and particularly to an apparatus and method for the time-gated optical examination of objects that are part of, embedded in or viewed through dense scattering materials using coherent anti-Stokes Raman scattering.
2. Description of the Related Art
Images of objects that are part of, embedded in, or viewed through a medium in which a significant amount of multiple path scattering occurs are usually blurred or otherwise degraded in resolution or completely obscured because the different paths over which the scattered radiation travels causes the image to appear to arise from more than one location within the scattering medium. FIG. 1 illustrates a scattering medium illuminated by a light beam 60. When the light beam 60 travels through the scattering medium 10, the light is scattered and emitted as beam 65. There are several methods that can be used to overcome this problem, with each having certain disadvantages.
A first method involves the spatial filtering of the image to include only those rays that are not deviated too far from the axis of the optical system. The limitations of this first method occur because the spatial frequencies of the scattered radiation can overlap those required to form the image of the object. Either the resolution with which the object can be imaged is limited, or the scattered radiation cannot be filtered out effectively.
A second method of overcoming the blurring of the image is to time gate the transmitted signal so that only the earliest light that emerges from the scattering medium is recorded by a detector. This "first light" either is not scattered, or is scattered over a relatively short path compared to light that emerges later, and therefore provides the least amount of image degradation. The degree of improvement provided by this second method depends on the length of the scattering path over which the detected signal is integrated, which, in turn, is determined directly by the duration of the time gate. In general, the shorter the time gate, the better the image, down to some characteristic time that is determined by the scattering characteristics of the medium. Imaging through dense scattering materials, such as biological tissue, or solids or liquids that appear translucent or even opaque to the unaided eye can require gating times of the order of 10 picoseconds or less.
There are several techniques currently used to perform such time-gated imaging measurements, including various forms of electronic gating and optical gating. Electronic gating can be accomplished either by gating a photoelectric image tube directly, or by switching some other part of the photoelectric detection circuit. These techniques are currently limited to gating times of the order of 50-100 picoseconds (psec) or longer, corresponding to minimum scattering paths of the order of 1.5-3 centimeters (cm) by the limitations of available electronic switching devices.
Another technique involves the use of picosecond or femtosecond pulses for illumination of the object, followed by an optical gating technique to provide the time resolution. Such techniques can provide time gates in the picosecond or subpicosecond regime, depending on the length of the optical pulse. For comparison with electronic gating methods, a time gate of 100 femtoseconds corresponds to a scattering path of 0.003 cm.
One gating technique suitable for picosecond or femtosecond pulses is holography, in which the image is detected only by a coincidence between the illumination pulse and a reference pulse of the desired length. Conventional holography, in which the image is recorded on high resolution photographic film, requires a substantial amount of light in the transmitted signal to interfere with the reference pulse to establish the holographic record. It thus limits the extinction in the sample that can be accommodated. Electronic holography, in which the fringes are detected with a sensitive two-dimensional camera and the hologram is reconstructed through computer analysis, overcomes the sensitivity problem, allowing greater attenuation in the sample. However, all of the transmitted light is recorded at the detector. If a large fraction of the transmitted light is contained in the non-image bearing tail that is delayed through scattering, the interference fringes that form the hologram will be washed out, and the noise in the image will be increased until the image is totally obscured.
Holography can also be accomplished with broad-band, long-pulse laser light, in which the gate time is determined by the inverse of the bandwidth of the light. This approach provides subpicosecond gate times without the need for subpicosecond technology. However, as it has been applied to date, it suffers from the same disadvantages described above for picosecond holography: large signal requirements and relatively low contrast between the image-bearing portion of the transmitted light and the non-image-bearing tail.
Another technique for short pulse gating is the use of a Kerr shutter, in which the transmission of light through a cell between crossed polarizers is controlled by a second pulse of light. The gate times for this approach can be of the order of picoseconds, depending on the duration of the controlling light pulse and the response time of the active medium in the Kerr gate. This technique suffers from limitations in contrast because of leakage of the wrong polarization through the polarizers, and losses in the Kerr gate because the transmission is less than 100%. Contrast can be increased by cascading gates, but only at the expense of overall transmission. The loss of transmission can be especially detrimental for viewing through highly attenuating samples in which there is a limit on allowable irradiation levels, such as for living tissue.
Image amplification with picosecond time-gated amplifiers have also been described in the prior art. These amplifiers have been based on dye amplifiers pumped by picosecond laser pulses. By themselves the dye amplifiers have relaxation times of the order of several nanoseconds and, therefore, gating times of the same order of magnitude. Picosecond gating times were achieved by raising the dye concentration and pumping level to such a degree that substantial radiation from the upper laser level occurs, leading to population "dumping" and reduced lifetime of the upper state. The limitations of these amplifiers are that the high level of fluorescence necessary to produce the short gating time contributes a background on top of the amplified image, limiting the sensitivity and increasing the noise level. The amplifiers have had gains of only 100 to 1000, limiting the degree of contrast with the delayed light. Finally, fundamental considerations of the noise level of amplifiers show that the minimum noise level occurs when the time-bandwidth product .DELTA..nu..DELTA.t=1. The dye fluorescence is radiated over the full bandwidth of the dye amplifier, of the order of 500 cm.sup.-1 . As a result, for gating times of the order of 10 picoseconds, the time-bandwidth product is in excess of 100, increasing the minimum noise value by the same factor.
Several other techniques are also possible. Streak cameras can be used to record the image. Time resolutions down to 2 picoseconds are currently possible. However, only a one-dimensional image is obtained, requiring scanning to produce a two-dimensional image. In addition, the streak cameras are of limited sensitivity, limiting their utility in detecting low-level signals. Another approach that uses time-gating involves the technique of four-wave mixing. In this approach the signal beam impinges on a non-linear medium that is being irradiated with co-propagating picosecond light pulses. Conversion of the signal light takes place only while the gating pulse is present. The main drawback to this approach is the combination of low conversion efficiencies associated with the conversion process (10% or less), coupled with limitations on the allowable illumination signal as set by the ANSI standards for irradiation of living tissue. Four-wave mixing using phase conjugation has also been suggested. The disadvantage of this technique is that, while phase conjugation can correct refractive distortion, it does not correct for scattering distortion due to fundamental considerations.
Non time-gating techniques also include the use of holographic recordings using spatial correlation to discriminate against the non image-bearing light. This approach has the same limitations due to low contrast with non-correlated light as discussed above for holography. Finally, use may be made of absorption in the sample to attenuate the longer paths associated with the multiple scattered light. This can work in materials that are highly absorbing, but not for materials that are primarily scattering rather than absorbing.
A recently developed system involves time gating by stimulated Raman amplification using short light pulses.